1. Field of the Invention
The present invention relates to a high-voltage capacitor and a magnetron having a filter constituted of the high-voltage capacitor.
2. Description of the Related Art
Well-known examples of high-voltage capacitors of this type in the prior art include those disclosed in Japanese Unexamined Patent Publication No. 1996-316099 and Japanese Unexamined Utility Model Publication No. 1992-40524. They have the following structural features in common. Two through holes are formed over a distance from each other at a dielectric ceramic material to constitute the capacitor. Individual electrodes that are independent of each other and a common electrode to be shared by the individual electrodes are provided at the two surfaces of the dielectric ceramic material at which the through holes open. The common electrode is secured onto a raised portion of a grounding member by a means such as soldering. Through conductors are provided so as to pass through the through holes at the capacitor and through holes formed at the grounding member. The through conductors are soldered to the individual electrodes at the capacitor by using electrode connectors or the like. An insulating case is fitted around the external circumference of the raised portion of the grounding member so as to enclose the capacitor. An insulating cover is fitted on the other side of the grounding member so as to enclose the through conductors. The insulating cover is mounted so that it comes in complete contact with the internal circumferential surface of the raised portion of the grounding member. Then, a thermo-setting insulating resin such as an epoxy resin is charged to fill the space inside the insulating case and outside the capacitor enclosed by the insulating case to assure satisfactory moisture resistance and insulation.
This type of high-voltage capacitor is crucial in application as a filter of the magnetron in a microwave oven, and, thus, since it is often used in a humid, dusty environment, it is required to achieve full voltage withstand performance in a humid environment.
When a high-voltage capacitor is mounted in a magnetron, the grounding member is grounded, and a high-voltage of, for instance, approximately 10 kV is applied to the through conductors. Since the insulating case is present within the path extending from the through conductors to the grounding member, the insulating case is subject to the high-voltage. As a result, the insulating case, too, is required to achieve full voltage withstand performance in a humid environment. The insulating case also must achieve properties such as fire resistance, tracking resistance, toughness and water repellency.
Materials that satisfy these performance requirements and are used to constitute the insulating case in the prior art include polybutylene terephthalate (PBT), polyethylene terephthalate and modified melamine. However, they still fail to achieve full voltage withstand performance in a humid environment.
In addition, it is desirable that the insulating case of this type of high-voltage capacitor has a small height since the high-voltage capacitor must be mounted in a magnetron. However, if the height of the insulating case is reduced, the creeping distance extending from the through conductors to the grounding members via the surface of the insulating case also becomes reduced to result in lowered voltage withstand performance in a humid environment.
It is an object of the present invention to provide a high-voltage capacitor achieving outstanding voltage withstand performance in a humid environment and a magnetron provided with a filter constituted of the high-voltage capacitor.
It is a further object of the present invention to provide a high-voltage capacitor having an insulating case with a small height and achieving outstanding voltage withstand performance in a humid environment and a magnetron provided with a filter constituted of the high-voltage capacitor.
In order to achieve the objects described above, the high-voltage capacitor according to the present invention comprises at least one grounding member, at least one capacitor, at least one through conductor, at least one insulating tube, at least one insulating case, at least one insulating cover and an insulating resin.
The grounding member is provided with a raised portion at one surface thereof, with at least one through hole passing from the one surface to another surface and formed at the raised portion. The capacitor includes a dielectric ceramic member having at least one through hole and is constituted by providing electrodes at the two surfaces of the dielectric ceramic member at which the through hole opens with one of the electrodes connected to the grounding member to achieve electrical continuity.
The through conductor passes through the capacitor and the grounding member and is connected to the other electrode to achieve electrical continuity. The insulating tube covers the through conductor.
The insulating case is provided at the one surface of the grounding member. The insulating cover is provided at the other surface of the grounding member. The insulating resin fills the space inside the insulating case and the space inside the insulating cover and also fills the space around the capacitor.
The insulating case is constituted of a material which is a mixture of polybutylene terephthalate and an inorganic material. The inorganic material, which contains glass powder and ceramic powder, is added so that its content is within a range of 15 wt % to 45 wt % relative to the entire quantity of the mixture.
When the high-voltage capacitor structured as described above is employed in the magnetron of a microwave oven, noise traveling through the through conductor can be absorbed through the filtering effect achieved by the capacitor by using the through conductor as a power supply terminal and connecting the capacitor between the through conductor and the grounding member achieving a ground potential.
In addition, the grounding member is provided with a through hole, and the capacitor is also provided with a through hole passing through the dielectric ceramic member, a through conductor with a higher potential relative to the potential of the ground can be installed between the grounding member achieving the ground potential and one of the electrodes at the capacitor while assuring a sufficient degree of electrical insulation achieved by the through holes.
Since the insulating resin fills the space around the capacitor, the reliability measured in reliability tests such as high temperature load tests and moisture resistance load tests and the reliability of the capacitor when it is utilized in a hot, humid environment are improved.
When a high-voltage capacitor is mounted in a magnetron, the grounding member is grounded, and a high-voltage is applied to the through conductor. Since the insulating case is provided within the path extending from the through conductors to the grounding member, the insulating case is subject to the high-voltage. As a result, the insulating case, too, is required to achieve full voltage withstand performance in a humid environment.
The insulating case in the high-voltage capacitor according to the present invention is constituted of a mixture containing polybutylene terephthalate and an inorganic material. The inorganic material contains glass powder and ceramic powder and its content relative to the entire quantity of the mixture is set within the range of 15 wt % to 45 wt %. Through testing conducted by the inventor of the present invention and the like, it has been confirmed that by adopting the structure described above in the insulating case, the humid-environment voltage withstand performance of the high-voltage capacitor is greatly improved.
The high-voltage capacitor according to the present invention having an insulating case structured as described above achieves superior voltage withstand performance in a humid environment as explained above. Thus, even if the height of the insulating case is reduced and, as a result, the creeping distance extending from the through conductor to the grounding member via the surface of the insulating case, too, becomes shorter, outstanding voltage withstand performance in a humid environment is assured.
In a desirable application, the insulating case has a height of 12 mm or greater with one end thereof inserted in the external circumferential side of the raised portion. Through testing conducted by the inventor of the present invention and the like, it has been confirmed that the required voltage withstand performance is assured even in a humid environment by setting the height of the insulating case to at least 12 mm.
In addition, in a desirable example of the high-voltage capacitor, the through conductor is constituted of a molding formed in the shape of a round bar with a tab portion formed by press-machining the round bar. In a through conductor adopting this structure, a connecting portion such as a caulked joint to connect the through portion, which passes through the capacitor, to the tab portion is not necessary. As a result, a high-voltage capacitor having an insulating case with a small height is realized.